Motion on a ship's roll axis can have several detrimental effects including cargo damage, reductions in crew effectiveness and increased pilot workload in helicopter landings. A maximum of 6° rms roll angle has been quantified for light manual work. Methods to attenuate this effect include the usage of fin stabilizers, bilge keels, anti-rolling tanks and rudder roll stabilizers (RRS). In contrast to other methods of roll motion reduction, RRS is attractive in that it does not require modifications to the vessel. Drawbacks of RRS have included the lack of performance at low speed, the need for a high speed rudder mechanism and the feedback limitations of the roll control loop. For an RRS system, the rudder is the actuator in a two output (roll and heading) system coupled by rudder-induced sway. Thus, the yaw and roll loops are designed with sufficient bandwidth separation, which may have a limiting effect on currently available roll control feedback. The roll plant is typically non-minimum phase, a characteristic in this application that increases the sensitivity of the closed loop system at low frequencies. The greatest limitation is the rudder mechanism itself, which is limited in maximum angle and angle rate. Several automated gain tuning algorithms to improve the performance of rudder roll stabilization controllers in saturation have been suggested, including the Automatic Gain Controller (AGC) and the Time-Varying Gain Reduction (TGR) algorithms. Model predictive control has also been applied to the rudder roll problem.
State of the art rudder roll stabilizers are typically proportional-derivative (PD) type, which provide marginal performance but retain stability when the rudder is saturated. A high-order rudder roll stabilizer with nonlinear dynamic compensation (HO+NDC) may provide substantially more roll reduction for ships having fast rudders (for example, 20°/s); however, rudder rate saturation may cause instability for such systems.